Uncooled thermal detectors have become an indispensable sensor technology in long-wavelength infrared (LWIR; 8 to 14 μm) imaging applications. Uncooled thermal detectors have sensitivity and resolution sufficient to replace cooled detectors in many applications. However, the state-of-the-art uncooled sensor technology, namely, microbolometers that sense the temperature by measuring a voltage on a thermistor, has progressed slowly over the past few years. Uncooled bolometers are limited in format (e.g., <3M pixel) and sensitivity. And microbolometers are based on thermistors whose resolution and sensitivity has improved slowly over the past few years. In addition, bolometers generally require complex fabrication methods and tend to be very expensive. For example, the support legs of a typical microbolometer structure are both thermally insulating and electrically conductive, which limits achievable thermal resistance. These constraints negatively impacts material selection, pixel size, fabrication complexity, yield, and performance.
Liquid crystal (LC)-based detectors have been investigated for uncooled thermal imaging since the 1970s. A liquid crystal thermal imager senses temperature by measuring the change in visible light flux through or reflected by a liquid crystal cell caused by changes in the state of the liquid crystal birefringence with temperature. As the liquid crystal cell changes temperature, its birefringence changes, which in turn causes a change in the polarization state of the incident visible light. Transmitting the visible light through a polarizer (aka an analyzer) transforms the change in the polarization state into an intensity change that can be measured with a conventional visible light detector, such as a charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) array. A sensitive liquid crystal thermal image can detect small changes in temperature in the scene (e.g., 30 mK with a pixel size of about 17 μm, an f/1 lens, and a frame rate of 30 Hz) which is synonymous with being able to detect small changes in the infrared light flux.
Liquid crystals have a relatively large change in birefringence with temperature (high sensitivity), easily adjusted composition (e.g., properties such as time response, birefringence, and noise can be modified by synthesizing different types of liquid crystals), and can leverage an extensive manufacturing base created for displays. To date, however, LC-based detectors have not been successfully integrated into a detector array that has good response in the LWIR region of the electromagnetic spectrum. The difficulties include fabricating small pixels and filling them with liquid crystals. In addition, the liquid crystals themselves tend to be noisy, which degrades detector performance.